Cancer patients' responses to clinical trial treatments are often hard to define and are becoming more so as an increasing number of novel treatment agents (e.g., cytostatic agents) are developed. Objective clinical endpoints often fail to capture the full impact of treatment on patients. Yet patients' symptoms can have a direct impact on patients' daily life, general well-being, and even adherence to treatment. Thus, physicians are looking to patient-reported outcomes (PROs). However, traditional methods of collecting PROs (e.g., retrospective recall) are limited by reliability and validity issues. The use of Ecological Momentary Assessment (EMA) devices (i.e., electronic daily diaries) addresses many of these issues by providing physicians with patients' own reports of daily changes in treatment-relevant outcomes such as quality of life. We propose a longitudinal, daily diary study to: (a) better understand intra- and inter-individual variability in patient-reported quality of life (PRQOL) for patients on Phase II clinical trials for prostate cancer, and (b) investigate how physicians use PRQOL information to make treatment decisions. Our pilot work shows that clinical trial patients, even those with stage 4 disease, are able to use EMA devices to complete daily PRQOL reports over extended periods of time with high completion rates. In the current study, our first objective is to evaluate the feasibility of using electronic diaries in a larger, more racially diverse sample of patients. To accomplish this goal, we will use EMA devices to collect daily PRQOL from 60 prostate cancer patients who are participating in Phase II treatment trials at an urban, comprehensive cancer center. Our second objective is to identify correlates of inter- and intra-individual variability in PRQOL and gain a better understanding of why patients may differ in their reports of QOL over time. Our third objective is to conduct a pilot intervention in which we use a two-condition, interupted times-series design to manipulate the type of patient QOL data available to physicians and examine whether the type of QOL data available in the different conditions affect physicians' use of patient QOL in decisions to stop or continue treatment. To do this, we will use a policy capturing methodology to estimate the relative weight that physicians assign to subjective PRQOL data and objective clinical data in their treatment decisions for prostate cancer patients in Phase II treatment trials. The significance of this study is in better understanding how clinical trial treatment impacts PRQOL and how physicians use PRQOL in treatment decisions. The innovative aspects of this study are (a) the use of EMA devices to collect PRQOL in response to treatment, and (b) the manipulation of the type of patient QOL data (self-reported by patients or physician-observed) available to physicians during clinic visits. The results of this study will inform the development of interventions to facilitate how PRQOL is collected and assist physicians in integrating PROs with objective clinical data, thereby leading to more informed and participatory treatment decision-making between patient and physician.